Immunology and Diagnostics
What the domain covers
Immunology and diagnostics tests innate and adaptive immunity, antigens and antibodies, immunoglobulin classes, humoral versus cell-mediated responses, vaccines, hypersensitivity, and diagnostic methods such as ELISA, agglutination, immunodiffusion, and Western blotting.
Core rules and concepts
- Innate immunity is rapid, non-specific, and lacks memory; adaptive immunity is specific and has memory, mediated by B and T lymphocytes.
- Antibodies are immunoglobulins with two heavy and two light chains. IgG is the main serum antibody and crosses the placenta; IgM is a pentamer and appears early; IgA is secretory; IgE is linked to allergy; IgD is a B-cell receptor.
- Humoral immunity involves B cells and antibodies; cell-mediated immunity involves T cells and is important against intracellular pathogens and tumour cells.
- Active immunity develops after infection or vaccination; passive immunity is transferred, as with maternal IgG or antiserum.
- ELISA uses enzyme-linked antibodies and a substrate to produce a colour signal; sandwich ELISA detects antigen, indirect ELISA detects antibodies.
Worked example
A serum sample is added to wells coated with viral antigen, then enzyme-linked anti-human antibody and substrate are added. A colour develops only if the serum contains specific human antibodies. Reasoning: the antigen captures antibodies; the second antibody detects human immunoglobulin; the enzyme converts substrate to a coloured product. Washing between steps removes unbound reagents.
Traps
Do not confuse active and passive immunity. Do not say ELISA necessarily confirms infectivity; it detects antigens or antibodies. Do not call IgM the main placental antibody-IgG crosses the placenta. Do not treat Western blot and ELISA as identical: Western blot separates proteins by size before detection.